Initially motorcycles used the bicycle's simple steered fork, a system which carries the wheels between the "tines" of the fork, whose "handle" pivots in steering bearings above the tire. The axis of these steering bearings, if projected downward, hits the road ahead of the point at which the tire touches the road. Like a furniture caster, the rolling tire then naturally tends to trail back into line behind the steering axis if deflected.
The important variable in this caster effect, called trail, is measured on the road surface as the distance between the tire contact point forward to the projection of the steering axis. The steering axis may pass through the axle centerline or it may not. If it doesn't, the distance between the two is called offset. The steering axis is inclined backward at an angle called rake, and acting with the trail and the weight of the front tire allows the front wheel to fall naturally to the steering angle that will make the motorcycle hold its line.
A change in one variable effects all the others. Rake and trail geometry are interrelated. If one increases the rake, the trail increases with it. Trail strongly influences the stability of the motorcycle. More trail makes steering harder to deflect and yields increased high-speed stability but heavier steering. Reducing trail lessens the self-stabilizing effects but also lightens the steering and increases responsiveness to the riders input.
The balancing of rake, trail and offset is necessary for an engineer to design the specific handling characteristics desired for the motorcycle. The range of adjustments is fairly large with a telescoping steering mechanism.
However, steering mechanisms utilizing a telescoping front fork suspension have several limitations inherent in their design, especially when they are used on a high performance motorcycle. One limitation is the tendency for the telescoping front fork to compress under sudden hard braking. This compression reduces the available suspension travel thereby making the suspension less effective.
Another problem caused by hard braking is that the compression of the suspension shortens the effective wheelbase of the motorcycle.
Yet another inherent design limitation with the telescoping front fork is that they are mounted to the frame of the motorcycle at a point far from the point of contact of the tire with the road. The mounting usually consists of at least one bearing and, due to the great distance from the point of contact of the tire with the road, a large amount of leveraged force is applied to the bearings. This makes it necessary to increase the strength of the frame in the area where the telescoping fork is mounted to it. These problems caused by the compression of hard breaking and the large amount of force being applied to the area of mounting to the frame, are exacerbated by the newly developed tires having increased adhesion to the road surface.
In the prior art the most common solution to the above problems is the use of hub centered steering. Difazio hub steering is one example. In this mechanism the wheel rotates around a large hub, usually at least six inches in diameter, on a large bearing along the circumference of the hub. Steering is accomplished about the axis of a kingpin, or two ball joints, which are mounted inside the hub. One or two swing arms are provided which are attached to the hub at one end and extend backward toward the frame or engine and are pivotably attached thereto at the other end. The swing arms are usually parallel to the road surface and one end of the suspension system is attached to the swing arm. The other end of the suspension system is attached to the frame of the motorcycle. Because steering is accomplished about the kingpin, hub centered steering has the disadvantage of being limited in the amount of trail or caster that can be accomplished without enlarging the hub even further. The invention provided herein overcomes this limitation and allows the designer of the motorcycle more freedom in the amount of trail and caster that can be built into the motorcycle.
Another steering mechanism which does not uses a telescoping fork is called an Earls Fork or leading link suspension system. This system does not solve the above problem of a large amount of leveraged force being applied to the point of mounting of the wheel to the frame and suffers a further disadvantage of being even less rigid than the telescoping fork system.